The present invention relates to a device for reducing noise and vibration caused by rotation of rotor blades of a rotorcraft such as a helicopter, and for improving kinetic efficiency thereof.
In the field of rotorcraft, it is generally known that, by the rotation of rotor blades, blade tip vortices of air current are generated along the rotational tracks of tips of the rotor blades and aerodynamic noise and vibration of the rotor blades are generated by a rotor blade beating the blade tip vortex which has been generated by the preceding rotor blade.
In the prior art to reduce such noise and vibration caused by the blade tip vortex interaction, there has been suggested a rotor blade of which blade tip is shaped so as to restrict an occurrence of the concentrated blade tip vortex. The above mentioned rotor blade can restrict the occurrence of the concentrated tip vortex to a certain extent, however, it cannot exclude the occurrence of vortex itself, so that there still remains the problem of the blade vortex interaction.
Alternately, in Japanese Unexamined Patent Publications (Kokai) Nos. 5-345596 and 5-124589, mechanisms (a device for controlling boundary layers in the vicinity of the rotor blades; a special rotor blade) are disclosed, wherein each rotor blade has a number of small orifices formed on blade surfaces and nozzles formed at the blade tip. In these arts, it is contemplated that, in operation, the air sucked into the rotor blade through the small orifices is discharged from the nozzles toward the center of the blade tip vortex thereby decreasing the intensity of blade tip vortex and improving the aerodynamic performance of the blade. In the above mentioned mechanism, however, since the air is discharged from the rotor blade owing to the centrifugal force thereof, it is difficult to ensure that the discharging air is intense enough to blow away the vortex already produced. In such a case, the discharged air will be swirled into the vortex flow and the intensity of the tip vortex cannot be reduced. Also, even if the air discharged from the blade tip directs its course toward the center of the blade tip vortex, an additional power is required to overcome the Coriolis force of the air flowing in the blade.
In the above mechanism, since the nozzles are fixed to the blade tips, the direction of air discharged from the nozzle is also fixed in the rearward direction which is intended to inject the air into the center of the tip vortex along the track of the blades.
In practice, however, the position of the blade tip vortex and the intensity and size thereof, vary with the flight velocity, attitude and mode of the rotorcraft, and with the environmental wind conditions. Furthermore, the blade tip vortex, after being generated, grows successively in a conical shape, increasing its vortex core size gradually. Therefore, even if the air is ejected from the blade tip rearward along the track of the blade, it is not always possible to effectively supply the air toward the center of the vortex. That is, there still exists a problem in the above mentioned arrangement with the fixed position nozzles which cannot effectively reduce the intensity of the blade tip vortex.
Next, to reduce the noise and vibration caused by the interference of the blade tip vortex of the preceding rotor blade with the following rotor blade, there is a known method of controlling the pitch angle of the rotor blades. However, this is a more complicated method since the pitch angle is controlled at a high frequency by using heavy-duty steering actuators in the arrangement.
Besides the above mentioned prior art, there is a known method of alleviating vortex generated in the vicinity of blade tip by actuating a movable flap, which is arranged on the trailing edge of the rotor blade, upward and downward. However, this method causes the structure of the rotor blade to be complicated, resulting in many problems in terms of its aerodynamic characteristics, maintenance, reduction in strength and so on.